R. B. Smith

Canine tracheal blood flow after endotracheal tube cuff inflation during normotension and hypotension

Tracheal tissue damage associated with endotracheal intubation may be a direct result of high mucosal contact pressure (MCP) generated by the endotracheal tube cuff. Tracheal blood flow (TBF) was measured at MCPs in the normotensive and hypotensive (mean arterial blood pressure, 50 mm Hg) canine model. Control TBFs through the individual rings in contact with the endotracheal tube cuff ranged between 26.6 ± 2.7 and 44.5 ± 5.0 with a mean of 35.0 ± 2.5 mL·min−1·100g−1 during normotension, and 15.0 ± 4.9 and 22.5 ± 5.0 with a mean of 18.9 ± 0.9 mL·min−1·100 g−1 during hypotension. TBF was reduced significantly at all elevated MCPs in both groups. TBF also was measured during normotension and hypotension after cuff inflation to 15 mm Hg MCP at 1-h intervals for 3 h. TBF was reduced significantly from control to 14.9 ± 1.5 mL·min−1·100 g−1 after 1 h during normotension, and continued to decline to 6.1 ± 0.9mL·min−1·100g−1 after 3 h. During hypotension, TBF decreased significantly from control to 6.1 ± 0.6 mL·min−1·100 g−1 at 1 h and remained unchanged at 3 h. These findings suggest that even at 20 mm Hg MCP, significant reductions in TBF may occur. For prolonged endotracheal intubation, especially during hypotension, significant reductions in TBF may occur at even lower MCP.

Continuous Flow Apneic Ventilation

A study was designed to evaluate the adequacy of gas exchange during continuous flow apneic ventilation (CFAV) in dogs. Seventeen dogs (average weight 22.9 kg) were divided into three experimental groups. Group I (n = 7) was anesthetized, paralyzed and ventilated with air using intermittent positive pressure ventilation (IPPV) through a tracheal tube. The tube was removed and each main stem bronchus was cannulated with a 2.5 mm i.d., 4 mm o.d. polyethylene catheter using a fiberoptic bronchoscope. The tracheal tube was replaced to hold the catheters in place. Heated, humidified air was continuously delivered equally to each catheter. Total flows ranged from 8 to 28 1/min (0.4—1.4 1 + kg‐1 min‐‐1). Airway pressure (Paw) in the trachea did not exceed 2 mmHg (0.27 kPa). Adequate gas exchange in terms of arterial oxygen and arterial carbon dioxide tension (Pao2 and Paco2) was found after 30 min at flows greater than 16 l · min‐1. Group II (n = 7) was managed similarly to the first group, insufflating endobronchial air using the optimal flow of 1.0 1 · kg 1 · min‐1 obtained from Group I. CFAV continued for 5 h in all animals. Blood gas samples and measurements of systemic blood pressure, heart rate (HR), pulmonary artery blood pressure, pulmonary artery wedge pressure, cardiac output (Qt), and temperature were taken every 30 min. Group III (n = 3) was anesthetized similarly to the other groups. Pulmonary gas distribution was evaluated in relation to catheter placement using Xe133. Results showed significant differences between Paoj values during CFAV and IPPV; however, all animals were adequately oxygenated. During 5 h of CFAV, adequate CO2 elimination was achieved in all animals. There was no difference in PaO2, Paco2 and shunt fraction (Qs/C}t) with CFAV at 30 min and 5 h. Differences in HR, Qt, and systemic vascular resistance at 30 min and 5 h were related to the hypothermia during the developing course of experimentation. With the catheters above the carina, gas distribution studies demonstrated gas limited to the large airways with no peripheral distribution, resulting in low Pao2 levels and elevated Paco2 levels. Endobronchial catheters permitted gas distribution to the peripheral airways, and oxygenation and ventilation were normal.

Percutaneous transtracheal jet ventilation for cardiopulmonary resuscitation: evaluation of a new jet ventilator.

This study compared percutaneous transtracheal jet ventilation (PTJV) at a frequency (f) of 20/min, with high-frequency positive-pressure ventilation (HFPPV) at f of 60/min, and endotracheal intubation and intermittent positive-pressure ventilation (ET IPPV) at f of 10/min in apneic dogs. Fifty-four emergency medicine trainees (EMTs) attempted PTJV via a 14-gauge Angiocath attached to a hand-held jet ventilator, f of 20/min, and ET IPPV using an Ambu bag, f of 10/min. Twenty-nine other EMTs attempted cricothyrotomy using a prototype nonkinkable catheter (Arrow) and a new jet ventilator, Bronchovent, f of 60/min, equipped with a pressure sensor which stops ventilation at pressures greater than 20 cm H2O. Adequate oxygenation was achieved by all 3 groups, but only the HFPPV group avoided respiratory alkalosis. There was a higher equipment failure rate (catheter kinking and dislodgment) in the PTJV group. In the HFPPV group, the Bronchovents pressure-limiting sensor stopped ventilation when the catheter was kinked or out of position, reducing the extent of subcutaneous emphysema and barotrauma. With further catheter improvements, HFPPV Bronchovent may offer a safe and reliable method of ventilating patients during CPR in the field.

Continuous-flow Apneic Ventilation during Thoracotomy

Continuous-flow apneic ventilation (CFAV) by endobronchial insufflation of conditioned gas was evaluated in dogs during thoracotomy. In Group 1 (n = 6), dogs were anesthetized with pentobarbital (25 mg/kg). An endobronchial catheter (2.5 mm ID) was introduced into each mainstem bronchus using a fiberoptic bronchoscope and held in place by an endotracheal tube. Before the onset of CFAV (total flow 1.0 1 · kg-1 · min-1, the animals were paralyzed with pan-curonium bromide and muscle relaxation was monitored with a peripheral nerve stimulator. The CFAV delivery system consisted of a flow meter, air/oxygen blender, oxygen analyzer, heated humidifier, and ultrasonic spirometer. Blood gas values were measured after 30 min of spontaneous ventilation, and CFAV with: 1) closed chest, fractional inspired O2 concentration (FIO2) 0.21; 2) open chest, FIO2 0.21; 3) open chest, FIO2 0.21, continuous positive airway pressure (CPAP) 5 mmHg; and 4) open chest FIO2 0.4, CPAP 5 mmHg. This last combination resulted in a mean PaO2 of 113.1 ± 5.5 (SEM) mmHg and a PaCO2 of 35.0 ± 2.1 (SEM) mmHg. In Group 2 (n = 6), animals with open chests were ventilated with CFAV (FIO2 0.4 and CPAP 5 mmHg) for 5 h. Adequate oxygenation and ventilation were achieved. PaCO2 after 5 h of CFAV was 41.8 ± 1.9 (SEM) mmHg compared with 40.8 ± 1.9 (SEM) mmHg during spontaneous breathing. PaO2 after 5 h of CFAV was 138.1 ± 11.7 (SEM) mmHg. There were no significant changes observed in vascular pressures. Significant differences in other hemodynamic parameters were probably due to pentobarbital anesthesia. Adequate gas exchange can be achieved during CFAV in dogs with open chests for 5 h.

Long-term transtracheal high frequency ventilation in dogs

Long-term effects of high frequency percutaneous transtracheal ventilation (HFTV) have not been studied. The purpose of this study is to evaluate the cardiopulmonary effects of 24 h of transtracheal ventilation in dogs at a respiratory rate of 100/min.Four dogs were anesthetized with intermittent pentobarbital and paralyzed with pancuronium. Ventilation in the supine position was through a 14-gauge Angiocath introduced into the trachea through the cricothyroid membrane. A respiratory rate of 100/min was used at an Fio2 of 0.4 using a fluidic logic controlled ventilator. The inspiratory-expiratory ratio was 1:2 and tidal volume 70 ml. The driving pressure of the air-oxygen mixture was 50 psi. After 24 h, residual muscle relaxant was reversed and the animal allowed to recover.There was no significant change in the following parameters over 24 h compared to starting values: Pao2, Paco2, pH, aortic, central venous, pulmonary artery and pulmonary artery wedge pressures, heart rate, cardiac index (CI), stroke index (SI), left ventricular stroke work (LVSW), systemic vascular resistance (SVR), pulmonary vascular resistance (PVR), C(a)o2, oxygen consumption (Vo2), pulmonary shunt (Qs/Qt). A PEEP effect of 2.9–5.0 torr was maintained. All dogs recovered uneventfully. Three days after the experiment, blood gases of 2 dogs were normal. One dog was killed after 3 days; macroscopic and microscopic examinations of the upper and lower airway and pulmonary parenchyma were normal.Dogs can be ventilated for as long as 24 h using HFTV transtracheally at rates of 100/min without adverse cardiopulmonary effects.

Regional Organ Blood Flow during High-frequency Positive-pressure Ventilation (HFPPV) and Intermittent Positive-pressure Ventilation (IPPV)

The effect of high-frequency ventilation (HFV) on cerebral blood flow (CBF) at normal and elevated intracranial pressure (ICP) was compared with flows measured under the same conditions during intermittent positive pressure ventilation (IPPV). Renal, lung (bronchial artery supply), and cardiac blood flows also were measured during HFV and compared with flows observed during IPPV. Measurements were made in canines with stable hemodynamic variables and arterial CO2 and O2 tensions in the normal range. CBF during HFV was comparable to the CBF during IPPV. Following an increase in ICP to a mean of 44 ± 18 mmHg (SD), mean CBF decreased to 22.5 ± 11 ml · 100 g-1 · min-1 (SD) during IPPV and 21.7 ± 13.2 ml · 100 g-1 · min-1 (SD) during HFV. No statistical differences could be noted in regional or global flow as a function of ventilatory mode. Renal, lung (bronchial artery supply), and cardiac blood flows also showed no statistical variation between HFV and IPPV. Ventilator-synchronous fluctuations in ICP observed during IPPV were reduced during HFV at normal ICP and eliminated by HFV at elevated ICP.

Apneic Diffusion Oxygenation and Continuous Flow Apneic Ventilation. A Review

Evolution of life coincides with the most rapid rate of rise in atmospheric oxygen concentration during Devonian time. Oxygen is essential for life, but at the same time is a cellular poison if present in tensions considerably higher than those the particular cellular milieu has become adapted to. Animals exposed to low oxygen tensions breathe continuously, e. g. most species of fish obtain their oxygen from a continuous flow of water at low oxygen tensions. Evolution from water to land breathing generally is associated with intermittent breathing, mass movement of gas in one direction, i. e. inhalation and exhalation. Intermittent breathing is part of this evolutionary process and reduces the inspired oxygen tension from 150 mmHg (19.95 kPa) to approximately 100 mmHg (13.3 kPa) at the alveolar blood interface. Perhaps evolution from continuous ventilation to intermittent breathing may be protecting the body against the high atmospheric oxygen tension. Therefore, it is not surprising that mammals can have adequate oxygen uptake with apneic diffusion oxygenation (ADO) and continuous flow apneic ventilation (CFAV). Mechanical ventilation classic techniques, i. e. intermittent positive‐pressure ventilation and continuous positive‐pressure ventilation, employ ventilatory frequencies close to the resting breathing rate of the adult. Utilizing low‐compression patient circuits has made mechanical ventilation with higher frequencies possible. 60 to 400 breaths per min is used for high‐frequency positive‐pressure ventilation and high‐frequency jet ventilation, and up to 40 Hz is used for high‐frequency oscillation (HFO). The two extremes of artificial ventilation ‐ ADO, i. e. supply of O2 by continuous flow, and HFO, i. e. supply of O2 by oscillatory changes ‐ both primarily involve diffusion, even though convection (also secondary to cardiac oscillations) obviously is important in the process of gas exchange. Some recent experimental findings favor continued development and evaluation of CFAV as an additional alternative to artificial ventilation.

Synergistic Effect of Acidosis and Succinylcholine‐Induced Hyperkalemia in Spinal Cord Transected Rats

The effects of spinal cord transection and acidosis on succinylcholine (SCC)‐induced hyperkalemia were studied in Sprague‐Dawley rats. The effectiveness of pretreatment with subparalyzing doses (“self‐taming”) of SCC or with the cholinesterase inhibitor hexafluorenium bromide in preventing hyperkalemia was also studied. The increase in plasma potassium after administration of SCC (1 mg/kg) was found to be significantly increased 10 days after spinal cord transection. This potassium increase could not be prevented by pretreatment with either hexafluorenium (0.3 mg/kg) or subparalyzing doses (0.15 mg/kg) of SCC. Respiratory acidosis caused an increase in plasma K+ in both normal and in spinal cord transected rats. Acidosis had a synergistic effect on succinylcholine‐induced hyperkalemia. These findings support the clinical practice of not using succinylcholine in patients at risk of having a pathological sensitivity to SCC. Furthermore, SCC may be especially dangerous when administered to patients who are acidotic.

Effects of hypo-, normo-, and hypercarbia in dogs with acute cardiac tamponade.

The hemodynamic effects of changes in Paco2 during intermittent positive pressure ventilation (IPPV) were studied in nine dogs with acute cardiac tamponade. During steady state light thiopental anesthesia, measurements were performed during hypocarbia (24.0 ± 2.6), normocarbia (40.4 ± 2.4), and hypercarbia (56.8 ± 3.1 mm Hg; mean ± SD). The study was carried out at a standardized level of cardiac tamponade that gave a 60% reduction in cardiac output (CO) at normocarbia. Changes in airway pressure were avoided by adding CO2 to the inspiratory gas to obtain the desired Paco2. Hypercarbia increased pericardial pressure 2–4 mm Hg and significantly decreased CO. During hypocarbia CO increased as pericardial pressure decreased 3–6 mm Hg. These findings are the reverse of changes seen when tamponade is not present. The changes in pericardial pressure most likely influence myocardial tone and cardiac volume and, thus, CO. The results suggest that patients with cardiac tamponade requiring general anesthesia should not breathe spontaneously if there is any danger of respiratory depression and hypercarbia.

High frequency ventilation in dogs with open chests.

This study evaluated and compared the physiological responses to high frequency percutaneous transtracheal ventilation in dogs before and after median sternotomy thoracotomy. Standard intermittent positive pressure ventilation (IPPV) was established before and after high frequency rates (100 and 300 breath/min) with the chest closed and then after thoracotomy. Gas exchange as judged by arterial and mixed venous blood gases, and cardiac performance as judged by pulmonary capillary wedge pressure, pulmonary and systemic arterial pressures, vascular resistances, and cardiac index all remained clinically acceptable. Physiologically, ventilation in the open chest condition was adequate but was associated with a slight decrease in PaCO2 and a decrease in PaO2. Peak and mean airway pressures were similar for IPPV and high frequency modes of ventilation.